This invention relates in general to measuring dark decay in an electrophotographic imaging member and more specifically, to an apparatus and process for assessing the projected life of an electrophotographic imaging member.
In the art of electrophotography an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the imaging surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated area. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
The flexible photoreceptor belts are usually multilayered and comprise a substrate, a conductive layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer and, in some embodiments, an anti-curl backing layer.
Although excellent toner images may be obtained with multilayered belt photoreceptors, it has been found that as more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, reduced life would occasionally be encountered during extended cycling. Surprisingly, cycling of belts made up of identical materials but differing in overall belt size and use in different copiers, duplicators and printers exhibited different life spans where one of the causes of failure was dark decay. Moreover, belts from different production runs had different life spans when cycled to the point of dark decay failure in any given copier, duplicator and printer. Since photoreceptor properties can vary from one production run to another and also during cycling, copy quality in many machines is maintained by feedback control system which constantly adjusts the machine operating parameters to compensate for the variations in the dark decay electrical characteristic of any given photoreceptor. Thus, photoreceptor life is partially governed by the design of the control system and this leads to different life spans in different machines for the same photoreceptor where failure is due to unacceptable dark decay. However, even the control system of any given machine cannot compensate for variations in photoreceptor dark decay characteristics that extend outside the operating range of the control system.
In the production of electrophotographic imaging members, particularly in web form, the complex nature of the manufacturing process renders unpredictable electrical characteristics of the coated web from batch to batch and from month to month. For example, reduction of photoreceptor life due to changes in environment affects the installation or adjustment of new coating applicators or the initial use of a newly prepared batch of coating material for one of the many layers of the photoreceptors such as the hole blocking layer, charge generating layer, or charge transport layer are difficult to identify within a reasonable length of time subsequent to the point in time that the photoreceptor comes off the production line.
During production of multilayered belt photoreceptors, a test run is conducted on prepared photoreceptor test samples each time a major change is made to the production line. Examples of such major changes include the installation or adjustment of new coating applicators or the initial use of a newly prepared batch of coating material for one of the many layers of the photoreceptors such as the hole blocking layer, charge generating layer, or charge transport layer.
One technique for determining how many cycles photoreceptors from a specific production run will perform satisfactorily in a specific type of given copier, duplicator and printer is to actually cycle the photoreceptor in the machine for the entire life of the photoreceptor. Generally, it has been found that actual machine testing provides the most accurate prediction of how a photoreceptor from a given batch will behave over its lifetime. However, machine testing for photoreceptor life requires weeks of testing involving hand feeding of sheets by test personnel along with constant monitoring of the final quality of every sheet. Since machine testing for the life of a photoreceptor belt requires hand feeding of copy sheets over many days for a single machine test of a single photoreceptor belt, the machine testing approach can be extremely expensive and time consuming. Moreover, accuracy of the test results depends a great deal upon interpretations and behavior of the personnel that are feeding and evaluating the sheets. Further, since machine characteristics vary from machine to machine for any given model or type, reliability of the final test results for any given machine model must factor in any peculiar quirks of that specific machine versus the characteristics of other machines of the same model or type. Because of machine complexity and variations from machine to machine, the data from a life test in a single machine is not sufficiently credible to justify the scrapping of an entire production batch of photoreceptor material. Thus, life tests are normally conducted in three or more machines. Life tests in copiers, duplicators or printers are extremely time consuming, labor intensive and very expensive. Since a given photoreceptor may be used in different kinds of machines such as copiers, duplicator and printers under markedly different operating conditions, the life prediction based on the machine life test of a representative test photoreceptor sample is specific to the actual machine in which photoreceptors from the tested batch will eventually be utilized will not necessarily predict what the life of that same type of photoreceptor would be in another different type of machine. Thus, for a machine life test, the test would have to be conducted on each different type of machine. This becomes extremely expensive and time consuming. Moreover, because of the length of time required for machine testing, the inventory of stockpiled photoreceptors waiting approval based on life testing of machines can reach unacceptably high levels. For example, a batch may consist of many rolls with each roll yielding thousands of belts. Still further delays are experienced subsequent to satisfactory life testing because the webs must thereafter be formed into belts, packaged and shipped.
Cycling scanners have also been utilized for life testing of photoreceptors. These scanners are designed to simulate the cycling of photoreceptors in a copier, duplicator and printer by subjecting a test sample of photoreceptor to timed charge, expose and discharge cycles. Scanners do not utilize all of the stations in a completely operational xerographic machine. Thus, for example, test scanners normally involve electrical charging, imagewise discharging and flood erase steps omitting the development, transfer and cleaning steps. Unfortunately, these scanners have proved to be of little value in regard to the issue of photoreceptor dark decay life in actual machines. For example, very little correlation has been found between scanner detected change in the charging potential at the time of development under traditional constant current cycling and actual machine testing. Also, scanners have proved to be very slow for production line monitoring.
Another technique for determining whether a photoreceptor have sufficient life to justify further processing is to fabricate the photoreceptors into belts and actually test how well they perform in customers' machines. Feedback in the form of reports from customers or performance evaluation reports from repair persons in the field are not always reliable because the tests are not conducted under controlled conditions and the cause of failure may be due to factors other than electrical such as the dark decay properties of the photoreceptor. Reliance on field tests can result in extensive delays, and, if the performance is unsatisfactory, will understandably aggravate customers. Moreover, reports from repair persons can be difficult to interpret because belt life may be affected by the peculiarities of the given machine involved, other factors affecting belt life that are unrelated to the electrical factors tested by the process of this invention, and the like. Also, data input from repair persons in the field requires one to accumulate and interpret the input over a period of time. This long delay can result in the introduction of large numbers of defective photoreceptor belts into the field.
To avoid fabricating an entire roll of photoreceptor material prior to testing, one can fabricate only a small part of the roll and test the resulting photoreceptor. If the test sample performs well during the test, the entire roll can thereafter be coated. However, testing by means of prior art techniques can still result in long delays because such testing required large amounts of time. Life testing in scanners can require several days, while 2 to 3 weeks are required for life test in machines and several months in the field are necessary for life tests in customers' machines. Thus, a production line could stand idle until the favorable test results were received. Since the expected life of a photoreceptor is extremely important from the viewpoint of manufacturing rate, inventory size, customer satisfaction and numerous other reasons, there is a great need for a system for rapidly determining the service life of flexible belt photoreceptors before initiating a full stage production run of flexible belt photoreceptors.